In a radio communication system, pilot signals are sent for channel estimations and channel measurements during a data transmission. With the application of MIMO technique in the radio communication system, it is necessary to further identify different spatial channels so as to achieve spatial selectivity. Therefore, a plurality of orthogonal pilots is sent for identifying a respective spatial channel. With the technical development, the more antennae a transceiver uses, the more pilots are needed. In a LTE system, pilots of various antenna ports are configured as full-band and the pilots are sent at each sub-frame so as to achieve channel estimations and channel measurements.
However, in the LTE-A system, the number of antenna ports supported by downlink increases to eight. In order to limit the pilots, the downlink pilots are classified as CSI reference signal and demodulation pilots in SI phase, The demodulation pilots are user-specific, which are stream-based structures, while the CSI reference signal are neighborhood-specific, which sparse distributed in the frequency domain based on the structures of the antenna port. However, the prior arts failed to involve any solution for sending the downlink CSI reference signal currently.